All the living beings need sunlight to function, to be alive. The sun gives them energy, makes them 'happy and alive'. If you are growing some plant and put it away from sunlight, you may be disappointed in short time. They will die in short time. If you don't give them clean air, love and attention, than they will return in admiration. When some dark energy appears from somewhere and tries to destroy everything around, than there must be some great light energy that will save that world from falling apart. We should not allow that thing to happen. We suppose you can guess that such kind of threat is happening in today's game.